This invention relates generally to radio frequency circuits and more particularly to radio frequency circuits for selectively attenuating a signal fed thereto.
As is known in the art, attenuators are used for a variety of applications, for automatic gain control circuits, and in particular, in broadband temperature compensated microwave amplifiers for temperature compensation of gain over an operating range of temperatures. One type of attenuator often used is a programmable attenuator. A programmable attenuator provides a selected predetermined fixed attenuation in response to a set of signals fed thereto. A general technique for providing a programmable attenuator, such as those described in U.S. Pat. No. 3,765,020 and U.S. Pat. No. 4,121,183, employs field effect transistors to selectively switch passive elements such as resistors to provide a properly configured attenuation network to thereby provide, in response to a signal fed to such network, a predetermined attenuated output signal. While these attenuators are useful in certain applications, one problem associated with these types of attenuators is that such circuits are not particularly well suited for fabrication on a common substrate such as by using monolithic microwave integrated circuit techniques since a resistor ladder network is required to provide attenuation, and an impedance matching network is required for matching the attenuator to external circuits.